I have a related question on this topic, if you could have a look and perhaps give some feedback would be really helpful.

The properties for image height en width are set up in Deco CMS by the pull-down list in an number of fixed values (175, 200, 250, 300x300) but we'd like to give these images 100% width and auto height. This is the script part:
<div style="height: 200px; width: 200px;">
<img src="...jpg" class="first" alt="PRODUCT NAME">
</div>

I won't be able to provide you direct coding assistance but if you are working with a web developer you can let them know to keep the parent elements of the image in mind when setting the width to a percentage.